Ethanol Photoreaction on RuO - American Chemical Society

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Ethanol Photoreaction on RuOx/Ru-Modified TiO2(110)

S. Kundu,† A. B. Vidal,†,‡ M. A. Nadeem,§ S. D. Senanayake,† H. Idriss,§ P. Liu,† J. A. Rodriguez,† and D. Stacchiola*,† †

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Centro de Química, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela § Department of Chemistry, University of Aberdeen and School of Engineering, Robert Gordon University, Aberdeen, United Kingdom ‡

ABSTRACT: During the photochemical reaction of organic molecules on oxide surfaces, radicals are formed and participate in heterogeneous photocatalytic processes; however, understanding the mechanistic origins and the fate of such species under reaction conditions is difficult. In this work we carry out a combined experimental and theoretical study on the thermal and photochemical interaction of ethanol with RuOx/TiO2(110) surfaces. Ethanol dissociatively adsorbs on both TiO2 and RuOx/TiO2 surfaces forming ethoxide. Our DFT calculations indicate that the ethoxide formation is more exothermic on RuOx/TiO2(110) surfaces (ΔE= −1.61 eV) than on the clean rutile TiO2(110) surface (ΔE= −0.95 eV). Defect sites present on RuOx/TiO2 surfaces can dissociate part of the ethoxide to acetaldehyde even below 300 K, which can be further oxidized to acetate resulting in the reduction of the RuOx nanoparticles. Exposure to UV irradiation of the ethoxide covered surfaces in the presence of oxygen at 300 K resulted in considerable decrease in ethoxide species by conversion to acetate. It is found that the Ru/TiO2 system is more active for the photo-oxidation of ethanol to acetaldehyde than TiO2. A linear trend of the rate of acetaldehyde and carbon dioxide production from exposure to ethanol of Ru/TiO2 surfaces in the presence of O2 indicates that more surface sites are available for the adsorption of O2 than on bare TiO2 surfaces, possibly at the interface of the Ru metal nanoparticles and TiO2 surfaces, which facilitates the photo-oxidation.

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INTRODUCTION Ethanol has been proposed as a renewable hydrogen carrier for energy purposes.1,2 Hence, there has been growing interest in studying the interaction of ethanol with surfaces of model catalysts, which can liberate the hydrogen contained in ethanol. Among different catalytic processes, ethanol photoreactions on metal oxides are receiving major attention due to their potential as materials for clean hydrogen production.3,4 The conversion of ethanol to hydrogen in the presence of steam or O2 has been reported at relatively high temperatures.1 In comparison with thermal processes, photoreactions could operate at lower temperatures and the energy required for the process could be fully renewable. Recently, it has been reported that Au/TiO2 can be a fast and efficient catalyst for hydrogen production from ethanol opening up the possibility of using metal/metaloxide systems as promising catalysts for the ethanol photoreaction.3 There have been several studies on the photocatalytic reactions of organic compounds using Ru-doped TiO2 as a catalyst. In 1989, Sobczynski and coworkers reported the hydrogen production from a methanol and water solution on polycrystalline Ru/TiO2 photocatalysts.5 It was reported that a very low concentration of ruthenium (ca. 0.75 wt.%) was most active for hydrogen production. Wetchakun and coworkers prepared a series of photocatalyst with 0.1, 0.2, 0.5, 1.0, and 2.0 wt % Ru loading on polycrystalline TiO2 to study the effect of ruthenium loading on the photocatalytic activity.6 TiO2 doped © XXXX American Chemical Society

with 0.1% Ru presented the highest photocatalytic activity for photomineralization of formic acid.7 In 2006, Sasirekha and coworkers studied the photocatalytic reduction of CO2 on Ru/ TiO2 anatase in the presence of water.8 They monitored the photocatalytic activity by analyzing the possible reduction products of the reaction including formic acid, formaldehyde, methanol, and methane. The study revealed that Ru-doped TiO2 particles have higher photocatalytic activity than pure TiO2, and it was proposed that the driving force for the observed enhanced activity was charge separation due to the formation of a Schottky barrier at the metal−oxide interface. There has been no detailed study on the interaction of ethanol with ruthenium metal/metal-oxide-modified TiO2 surfaces and its photocatalytic activity toward ethanol decomposition. Model catalysts prepared on single crystal surfaces can offer a more methodical approach to understand the reaction mechanism and accurately monitor the effect of metal oxide surfaces on the reaction products of ethanol. Moreover, our previous study of RuOx 1D nanostripes deposited on TiO2 shows special chemical properties for CO oxidation and water dissociation in comparison with bulk ruthenium oxide.9,10 It is important to reveal the changes imposed on the electronic structure of TiO2 Received: February 12, 2013 Revised: April 26, 2013

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dx.doi.org/10.1021/jp4015367 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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sections. The photon flux at the crystal surface was 3.2 × 1016 photons/cm2 s, as calibrated using a photodiode; comparable to values reported in the literature.11,27 Exposure of the TiO2 (110) crystal to the photon flux resulted in a rise in the crystal temperature of no more than 5 K. Theoretical calculations were performed using the planewave DFT approach within the projector-augmented wave method (PAW)12 using the GGA exchange correlation functional proposed by Perdew et al.,13 as implemented in the VASP 5.2 code. Following a previous study, a plane-wave cutoff energy of 400 eV was used.14−17 We treated the Ti (3s, 3p, 3d, 4s), Ru (4d, 5s), O (2s, 2p), and H (1s) electrons as valence states, while the remaining electrons were kept frozen as core states. Thermal smearing of one-electron states (kBT = 0.05 eV) was allowed using the Gaussian smearing method to define the partial occupancies to obtain faster convergence. For bulk TiO2, we used the optimized lattice parameters reported in our previous work,9,10 which are in good agreement with experimental values (in parentheses): 4.593 (4.594) and 2.956 (2.958) Å. A (2 × 2) four-TiO2-layer thick model slab was used to describe the TiO2(110) surface, and the wire-like structure of Ru3O6 on TiO2(110) was taken from our previous study.9,10 The Ru3O6 features and the two top layers of the titania support were allowed to fully relax, while the two bottom layers were kept fixed to their bulk positions. The calculations were carried out using a (8 × 8× 10) mesh for TiO2 rutile bulk and (4 × 2× 1) for the Ru3O6/TiO2 slab. Transitions states have been calculated by using the climbing image (CI) version of the nudged elastic band (NEB) algorithm, and in all cases, after a vibrational analysis, a single imaginary frequency has been obtained for these structures.

by deposition of Ru metal/metal-oxide nanostructures to understand the ethanol photoreaction on this system. In this work, we have studied the modifications of the electronic structure of TiO2 (110) surfaces after depositing ruthenium metal/metal-oxide nanostructures using near-edge X-ray absorption spectroscopy (NEXAFS). The interaction of ethanol on these surfaces was then investigated under dark condition and under UV irradiation employing X-ray photoelectron spectroscopy (XPS) in the presence and in the absence of molecular oxygen. Gas-phase reaction on these surfaces under dark and UV conditions was monitored by online mass spectrometry. The photostability of the surface ethoxide species and the possible formation of reaction intermediates on these surfaces were studied in detail in combination with density functional theory (DFT) calculations.

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EXPERIMENTAL METHODS Clean TiO2 (110) surfaces were prepared by repeated cycles of argon-ion sputtering and annealing to 900 K in the presence of oxygen. Ru3(CO)12 vapor was introduced to the chamber by a doser to deposit Ru on TiO2 surface, raising the chamber pressure to 1 × 10−8 Torr.9,10 While dosing, the TiO2 crystal was at 300 K. X-ray photoemission studies were carried out at the beamline U12A at the National Synchrotron Light Source (NSLS) using photon energies in the range of 260−650 eV. Ethanol was cleaned using the freeze−pump−thaw process several times to remove any contaminations. Ethanol vapor was introduced by a doser connected to the chamber, raising the chamber pressure to 5 × 10−9 Torr. NEXAFS experiments were performed at the end station of beamline U7A at the NSLS, equipped with a partial electron yield detector. Partial electron yield intensities were measured with a single channeltron with a front-end bias of −250 V to reduce signals from secondary electrons. All NEXAFS data reported here were collected with the photon beam fixed at 55° from the surface normal to the sample. The NEXAFS spectra were measured as a function of the incident X-ray photon energy in the vicinity of the titanium L-edge (445−490 eV), the oxygen K-edge (520−590 eV), and the ruthenium M-edge (270−320 eV) regions. The energy resolution in the NEXAFS experiments was ∼0.1 eV. Temperature-programmed desorption (TPD) and UV experiments were conducted in two UHV chambers equipped with either a Hiden mass spectrometer or an ExTorr (XT200M) quadrupole mass spectrometer, enclosed in a Pyrex tube with a small aperture and with a base pressure below 5 × 10−10 Torr. UV experiments were conducted by irradiating the sample using a 150W Hg arc lamp. The light was focused onto the sample using a 0.4 mm diameter fused silica fiber optic cable that directed the light through a UHV compatible feed-through onto the crystal face. The use of a fiber optic system allows exclusively exposing the light onto the crystal surface and not its surroundings. Photodesorption measurements were performed with UV light incident to the crystal face at a 30° angle from the surface normal and with the crystal facing toward the entrance of the apertured mass spectrometer. Photon exposures were started and ended using a mechanical shutter situated on the lamp’s housing. Although there was a complete attenuation of the incident photon by the TiO2 crystal, possibly a part of the incident light was reflected from the crystal surface. However, the amount of light reflection was not accounted for during the photochemical cross section measurements because such a correction would not change the order of magnitude of the resulting cross

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RESULT AND DISCUSSIONS Electronic Properties of Ru/RuOx-Modified TiO2 Surface and Ethanol Adsorption. Ruthenium was deposited on TiO2 surfaces using Ru3(CO)12 as a precursor. Our previous studies show that at 300 K Ru3(CO)12 partially decomposed upon adsorption on TiO2(110).9 All of the Ru−CO bonds cleave when the sample temperature is raised to 600 K, resulting in metallic Ru with trace amounts of carbon left on the surface. The Ru particles in contact with TiO2(110) were very reactive toward molecular oxygen. Exposure to O2 at 550 K led to oxidation of ruthenium to ruthenium oxide. We have reported STM studies on these surfaces previously and shown that for a small coverage of Ru (less than a monolayer), Ru metal clusters are formed on TiO2 surfaces with average sizes between 2 and 5 nm.9 In contrast, ruthenium oxide forms unique wirelike nanostructures that are well-dispersed on the top of TiO2(110) and covers the substrate in a uniformly spatially distributed way. The center of a RuO2 nanowire is aligned with the cus-Ti rows of the TiO2(110) substrate, and this wire (or strand) has an apparent width of ∼9 Å and heights in the range of 2.2 to 3.0 Å (height of a single layer of RuO2).9 The electronic structure of RuOx/TiO2(110) surfaces was studied using NEXAFS under dark condition and under UV exposure. Figure 1 shows the O K-edge spectra from a rutile TiO2 (110) single crystal and for small coverages of Ru and RuO2 on the TiO2 surfaces under dark conditions (solid line) and under UV irradiation (circled line). Only slight changes in the O K-edge can be observed after depositing Ru/RuO2 on TiO2: There is a slight increase in the t2g to eg ratio from 1.1 (for clean TiO2) to 1.2, indicating an enhancement in the number of electrons in unoccupied d orbital states, which can B



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Figure 1. O K-edge NEXAFS of TiO2(110), Ru/TiO2(110), and RuO2/TiO2(110) surfaces obtained by partial electron yield measurements. The solid lines indicate the spectra taken under dark condition and the circled lines indicate the spectra taken under UV exposure.

be attributed to the presence of oxygen vacancies; In the presence of Ru as well as RuO2, the t2g and eg peak width increases slightly, indicating a dispersion of the LUMO due to the hybridization of O2p with the Ru 4s+4p. The dispersion of the LUMO facilitates the mobility of electrons into the conduction band, which is suggested to enhance the photocatalytic activity.18,19 Upon irradiation with UV light no changes were observed in the O K-edge for TiO2; however, a significant change was found for Ru/TiO2 and RuOx/TiO2, where the t2g to eg ratio increased to 1.3 and to 1.5, respectively. A broadening of the t2g and eg peak for RuO2/TiO2 indicated further enhancement in electron mobility in the conduction band under UV light. Hence, Ru/TiO2 and RuO2/TiO2 appear to be promising surfaces to perform photo reactions. Ethanol Reaction on TiO2(110) under Dark Conditions. Figure 2 shows C1s XP spectra obtained after ethanol absorption on TiO2 surfaces at full saturation at 100 K and subsequent annealing to higher temperatures. At 100 K physisorbed ethanol is present at the surface, as indicated by the two peaks observed in the C1s spectra at 286.4 eV (due to CH3) and at 287.9 eV (due to CH2OH).4,19,20 However, when heated to 175 K physisorbed ethanol is desorbed, and only chemisorbed ethanol remains on the surface at this temperature. Ethanol adsorption on TiO2(110) surfaces is mainly dissociative, thus forming ethoxide species.21−30 The position of the C1s peaks from 170 K onward confirmed the formation of ethoxide species deconvoluted into two peaks at 285.3 and 286.8 eV. Heating the surface to higher temperatures resulted in decreasing the surface ethoxide population due to thermal desorption and reaction. The decrease in peak intensity occurs almost at the same rate for both functional groups (−CH3 and −CH2O−), indicating that no other stable intermediate species are forming on the TiO2 surface at elevated temperatures. Almost all of the surface species are removed after heating the surface to 900 K, where traces of hydrocarbons are left. Figure 3 shows a representative TPD after ethanol adsorption at full saturation at 290 K. Two main desorption states can be observed: a low-temperature (LT) state (350 K500 K) and a high-temperature (HT) state (600−750 K), which are in agreement with previous results in the

Figure 2. XP C1s spectra obtained after heating a TiO2(110) surface presaturated with ethanol at 100 K.

Figure 3. TPD after ethanol adsorption at 290 K on a rutile TiO2(110) single-crystal surface with a heating rate 3K/s.

literature.21−26 TPD results are also in good agreement with XPS results, which indicate that all of the surface species are desorbed by 800 K. The main desorption products consist of ethanol (mass 31 and 29), acetaldehyde (mass 29), and ethylene (mass 28). In the LT state the main contribution is from ethanol. At the HT desorption state ethylene and acetaldehyde are also produced. As suggested in the literature, the LT desorbed ethanol is originated from recombination of C



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Figure 4. Structures for adsorption and reaction of ethanol on TiO2(110). Color code: red (oxygen), blue (ruthenium), gray (titanium), and white (hydrogen). (a) Before ethanol adsorption on TiO2(110), (b) ethanol adsorption on TiO2(110), (c) ethoxide formation, (d) η1-acetaldehyde formation, (e) hydrogen desorption, and (f) H2O desorption and oxygen vacancy formation on TiO2(110) surface.

Figure 5. (a) Solid lines () represent the XP C1s and Ru 3d spectra obtained after heating the RuO2/TiO2(110) surface presaturated with ethanol to 100 K and the circled line (−○−) presents the Ru 3d spectra of RuO2/TiO2 surface obtained before dosing ethanol. The inset figure compares the ethanol-saturated TiO2 and RuOx/TiO2 surfaces at 110 K. (b) XP C1s spectra of ethanol present on (i) TiO2 and (ii) RuO2/TiO2 surface at 300 K.

ethoxide species (on Ti4+ cations along the [001] direction)

CH3CH 2O(ads.) + OH(ads.)

with surface hydroxyls (of bridging oxygen atoms). The HT

→ CH 2CH 2 + H 2O + O(surface)

desorption is due to decomposition of ethoxide species into ethylene and acetaldehyde. The reactions that occur are:

Formation of acetaldehyde by dehydrogenation: CH3CH 2O(ads.) + OH(ads.)

Ethanol desorption due to a recombinative reaction:

→ CH3CHO + H 2 + O(surface)

CH3CH 2O(ads.) + OH(ads.) → CH3CH 2OH + O(surface)

(2)

(3)

We have investigated the interaction of ethanol with the stoichiometric TiO2(110) rutile surface using DFT calculations, as shown in Figure 4. Previous studies reported three modes of ethanol adsorption on surfaces: molecular, dissociated along the

(1)

Formation of ethylene by dehydration: D



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[001] direction, and dissociated along the [110] direction.24−32 In the case of ethanol on the TiO2(110) surface the dissociated form (ethoxy species) was found to be more stable than the molecular form, and the ethoxide formation is exothermic (reaction energy, ΔE= −0.95 eV) (Figure 4). This agrees well with our experiment, in which ethoxy species are observed at LT (Figure 2). Ethoxy species are adsorbed on five-fold Ti sites, and the dissociated H prefers to attach to the bridging oxygen (Figure 4) rather than the in-plane oxygen. This is in agreement with the previous work, showing the preference of ethoxide formation on TiO2 surfaces.25−32 In contrast, the formation of η1-acetaldehyde by dehydrogenation of ethoxide on the stoichiometric TiO2 surface is endothermic,33 accompanied by H2 (ΔE = 1.26 eV, Figure 4d,e) or H2O (ΔE= 2.62 eV, Figure 4d−f) as secondary products. The formation of the acetaldehyde could therefore take place on steps or defect sites of the titania surface. Ethanol Interaction with RuO2/TiO2(110) Surfaces under Dark Condition. Similar studies were carried out to investigate the interaction of ethanol with RuO2-modified TiO2(110) surfaces, where RuO2 nanoparticles covered ∼50% of the TiO2 surface. Figure 5a shows the effect of annealing to different temperatures on the population of surface species, after saturating the RuO2/TiO2 surface with ethanol at 110 K. The C1s spectra in the inset indicate that the ethanol uptake on bare TiO2 and RuOx/TiO2 surfaces at 110 K is almost the same. The C1s spectrum at 110 K shows two peaks at 286.4 and 287.9 eV, which can be attributed to physisorbed ethanol on RuO2/TiO2 surfaces. After heating to 200 K, 75% of the ethanol has desorbed, significantly different when compared with the clean TiO2, where only 20% of total alcohol has desorbed under similar treatment. Upon annealing to 250 and 300 K, three main changes are observed in the deconvoluted XP spectra (Figure 5b): there is a shift in the C1s peak toward lower binding energy, indicating the formation of ethoxy species associated with a gradual decay in the amount of ethoxy species with increasing temperature; there is a partial reduction of RuO2, as indicated by the formation of a Ru3+ peak;9,10 and formation of acetate can be observed at 300 K, as indicated by the peak present at 289.5 eV. When compared with the clean TiO2(110) surface, the formation of acetate at 300 K is unique to the RuOx/TiO2 surface. Figure 6 shows a representative TPD after ethanol adsorption at full saturation at 290 K on RuO2/TiO2 surfaces. Similar to the TPD on TiO2 surfaces, in this case, we can also observe two main desorption states: a LT state at 300−500 K and a HT state at 600−750 K. All surface species are desorbed by 800 K. The main desorbed products are similar to the previous masses during ethanol decomposition/reaction on TiO2(110) single-crystal surface, such as ethanol (mass 31 and 29), acetaldehyde (mass 29), and ethylene (mass 28). However, the amount of desorbed species from RuOx/TiO2 surface is significantly lower compared with TiO2 surfaces, and the ratio of products between the two peaks is tilted toward the HT state. The LT regime appearing at ∼400 K shows mainly desorption of unreacted ethanol. For the HT state emerging at 660 K, the main desorbed species are ethylene and acetaldehyde. DFT calculations were carried out to investigate the ethanol interaction with RuO2/TiO2(110) surfaces. The corresponding optimized configurations are shown in Figure 7, and the corresponding ΔE values for each intermediate step are also included. Our calculations show that similar to the case of

Figure 6. TPD after ethanol adsorption at 290 K on RuO2/TiO2(110) surfaces with a heating rate of 3 K/sec.

TiO2, ethanol adsorbs dissociatively on a five-fold Ru site on the RuO2/TiO2(110) surface (Figure 7a−c). Interestingly, the ethoxide formation is much more exothermic (ΔE = −1.61 eV) on RuO 2 /TiO 2 as compared with pure TiO 2 . More importantly, the dehydrogenation process of ethoxide to form η1-acetaldehyde is now exothermic on the RuO2/TiO2 surface (ΔE = −0.53 eV, Figure 7d). This is in contrast with bare TiO2 where the η1-acetaldehyde formation was endothermic {ΔE = 0.82 eV, Figure 4d}. Accordingly, a facile decomposition of ethanol to acetaldehyde on RuO2/TiO2 is expected even below 300 K. The dissociated H prefers to attach to the bridging oxygen (Figure 7d) rather than the in-plane oxygen of the RuO2 nanowire. Our previous studies indicate that in comparison with bulk RuO2, RuO2 nanowires deposited on TiO2(110) can get easily reduced to Ru2O3 or metallic Ru.9,10 Indeed, our DFT calculations predict the formation of oxygen vacancies during the dehydrogenation of adsorbed acetaldehyde to form η1-acetic acid. This is an endothermic process (ΔE = 0.75 eV) with the dissociated hydrogen adsorbing at the neighbor Ru site (Figure 7e). The endothermicity can be further released by the diffusion of the hydrogen atom adsorbed at the Ru site to the more stable bridging oxygen (ΔE= −0.45 eV), leading to the recovery of two hydroxyl species on the surface (Figure 7f). However, η1-acetic acid is not stable on the surface, and the decomposition to form bidentate acetate is highly exothermic (ΔE = −1.85 eV, Figure 7g), where the two oxygen atoms are attached to two five-fold Ru sites and the acetate formation is highly exothermic. Both hydrogen atoms dissociated during the formation of η1-acetic acid and bidentate acetate preferably attach to the terminal oxygen of the RuO2 nanowires at the interface of the RuOx nanoparticles and TiO 2 (110) surface. The result agrees well with our experimental results, where the reduction of RuO2 and the presence of acetate on RuOx/TiO2 surfaces below 300 K are observed. We also take into consideration the fact that similar to the ethanol interaction on TiO2 surfaces, at high temperature acetaldehyde can desorb directly from the RuO2/TiO2 surface along with hydrogen and/or water (Figures 7h,i) rather than E



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Figure 7. Geometries for the adsorption and reaction of ethanol with RuO2/TiO2 (110) surfaces. (a) Before ethanol adsorption, (b) after ethanol adsorption, (c) ethoxide formation, (d) η1-acetaldehyde formation, (e) η1-acetic acid formation and reduction of RuO2, (f) hydrogen diffusion, (g) acetate formation, (h) hydrogen desorption, and (i) water desorption. Color code: red (oxygen), blue (ruthenium), gray (titanium), and white (hydrogen). Green arrows represent the preferred pathway, while the red arrows label the highly activated pathway.

Figure 8. (a) Acetaldehyde desorption from ethanol-saturated RuOx/TiO2(110) surfaces at 300 K under UV light exposure and in the presence and absence of O2 as indicated. (b) C1s XPS of an ethanol-saturated RuO2/TiO2(110) surface (i) before and (ii) after UV exposure and (iii) after UV exposure on an ethanol-saturated TiO2(110) surface. The surface was exposed to UV light for 5 min under O2 pressure of ∼1 × 10−8 Torr.

the surface. All of the reaction steps are favored on RuO2/TiO2 due to the strong RuO2−TiO2 interaction, the metallic character of RuO2 and the active low-coordinated sites at the interface, which facilitates electron and proton transfers. Ethanol Photoreaction. Figure 8a presents spectra of the acetaldehyde desorption from different surfaces saturated with ethanol at 300 K under UV light exposure and in the presence

forming acetate. Our DFT calculations indicate that both hydrogen and water desorption from the RuO2/TiO2 surfaces is highly endothermic (ΔE = 1.96 and 2.10 eV). Hence the DFT calculations indicate that the deposition of RuO2 nanowires on TiO2(110) varies the catalytic behavior of TiO2(110) toward ethanol. RuO2/TiO2 favors the formation of acetate and prevents desorption of hydrogen and water from F



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Figure 9. Plot (a) of Ln[CH3CHO] and (b) of Ln([CO2]) as a function of Ln[PO2] under UV excitation of the ethanol/ethoxide-covered Ru/ TiO2(110) surface at 300 K.

intermediate during the ethanol photo-oxidation on our surfaces. It is clear that the presence of RuO2 on TiO2 surface enhances the activity toward ethanol photoreaction. However, once the RuO2/TiO2 surface is exposed to ethanol the Ru4+ gets reduced to Ru3+, as evidenced by XP spectra.9,10 Hence, we also measured acetaldehyde desorption spectra on this partially reduced (Ru3+) and fully reduced (Ru0) ruthenium nanoparticles-modified TiO2 surfaces under UV exposure in UHV, as shown in Figure 8a. It is evident that there is a gradual decrease in the amount of acetaldehyde desorption with a decrease in the oxidation state of ruthenium. Ru/TiO2 was the most stable surface, and there was no decay in the activity after consecutive dosing of ethanol and photodesorption of acetaldehyde from the surface. Further studies of acetaldehyde production were carried on Ru/TiO2 surfaces that were presaturated with ethanol at 300 K as a function of O2 exposure. The oxidation of acetaldehyde in an O2 atmosphere can be described by the following simplified consecutive reaction sequence:

and absence of O2, as indicated. When UV light is irradiated on a clean TiO2 rutile surface that has been predosed with ethanol, only a very small amount of acetaldehyde desorption was detected. In comparison, when an ethanol-saturated RuO2/ TiO2 surface was exposed to UV irradiation, a significant amount of acetaldehyde was desorbed from the surface. It has been previously shown on clean TiO2 surfaces that to make the photo reaction more efficient the presence of electron traps is needed to reduce the electron−hole recombination. This can be done by adding oxygen while the surface is exposed to the UV irradiation.27 It is clear that the addition of 1 × 10−7 Torr of oxygen increased the acetaldehyde formation on RuO2/TiO2 surfaces, as shown in Figure 8a. Acetaldehyde desorption is only a part of the reaction as acetate species are also formed by further oxidation. The formation of acetate in the presence of oxygen under UV irradiation was confirmed by XP C1s spectra taken under these conditions. Trace (i) in Figure 8b shows the XP C1s spectra obtained after saturating the RuO2/TiO2(110) surface with ethanol at 300 K. A trace amount of acetate is already formed on RuOx/TiO2 surfaces, as indicated by the small acetate peak present at 289.5 eV. Trace (ii) in Figure 8b indicates that a significant amount of acetate is formed when the ethanol-saturated RuO2/TiO2 surface was exposed to oxygen (PO2 ≈ 1 × 10−7 Torr) at 300 K under UV light for 5 min. The formation of acetate on clean TiO2 under these conditions is also shown in Figure 8b, trace (iii). A close look to the acetate peak indicates that after 5 min a slightly higher amount of acetate is present on RuO2/TiO2 surfaces compared with clean TiO2 surfaces. The photo-oxidation of acetaldehyde on rutile TiO2(110) surfaces has been previously reported.34 It was proposed that acetaldehyde reacts thermally with chemisorbed oxygen on TiO2(110) surface to form a photoactive acetaldehyde−oxygen complex, and upon exposure to UV the complex dissociates into gas-phase methyl radicals and surface-adsorbed formate species. Our TPD study indicates that during photo-oxidation of ethanol on TiO2(110) surface the mass 15 (CH3) exactly follows the pattern of mass 29 (CHO), thus confirming the absence of methyl radical formation. The results from Henderson et al. indicate that no methyl radical was observed from irradiation of η1-acetaldehyde adsorbed on UHV annealed TiO2(110) surfaces,34 in agreement with our results. In the case of photo-oxidation of ethanol on RuO2/TiO2 surfaces, no methyl radicals were detected during TPD, and thus we can exclude the acetaldehyde−oxygen complex as a possible

θ1k1

CH3CHO(ads) + O2−• ⎯→ ⎯ CH3COO(ads) + OH(radical) θ2k 2

⎯⎯⎯→ CO2 + CH4 + O(ads) + h+

(4)

where θiki are surface coverage and rate constant, respectively. In this equation an oxygen molecule captures one photoelectron from the conduction band and reacts with acetaldehyde to form acetate species and an OH radical. These may further react to give CO2 and CH4 injecting one electron into the valence band, as previously proposed.27,28 These previous studies show that surface acetate species react via the “photo-Kolbe”-type reaction to give CO2 and methane/ ethane as the main products. This hypothesis was further confirmed by the desorption of acetaldehyde and carbon dioxide when ethanol-saturated Ru/TiO2 surfaces were exposed to UV irradiation at 300 K in the presence of oxygen. By quantifying the amount of acetaldehyde formation as a function of time at different O2 pressures, it is observed that the rate of acetaldehyde formation increases with increasing O2 pressure (Figure 9a). The reaction rate for acetaldehyde formation under UV excitation and O2 pressure can be expressed by the following equation: r = k[ethanol]a [O2 ]b G

(5)



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accelerating the electron transfer process. While XPS C1s after photo-oxidation of ethanol exposed surface at saturation showed considerable amount of acetates (Figure 8), the small amount of CO2 formed (Table 1) indicates that the decomposition pathway is not much affected by the presence of RuO2 (low k2). Indeed, TiO2 alone (in the presence of O2) under UV is active for the photo-Kolbe reaction, a reaction that is initiated by hole trapping for the carboxylic species resulting in the desorption of CO2 and the association of the resulting CH3 radical with a hydrogen radical to make CH4. It is to be noted that this ratio can also be affected by the residence time of the reaction intermediates, where it was observed that incorporating TiO2 in the form of nanoparticles into carbon molecular sieve fibers41 dramatically changes the reaction selectivity (k1/k2) compared with bare TiO2. In a very recent study it has been reported that benzyl alcohol is selectively photo-oxidized to benzaldehyde on Ir/ TiO2 catalysts in the presence of molecular oxygen11,35 It was postulated that under UV irradiation the dissociatively adsorbed benzyl alcohol molecule adsorbed on TiO2 reacts with photogenerated holes to produce carbon radical (C6H5C•HO−). The reaction of this radical with chemisorbed oxygen on Ir results in the formation of benzaldehyde. On the basis of these observations and on our previous studies on TiO2(110) surface alone,20 the following reaction mechanism can be postulated:

Because only a small part of ethoxide at saturation coverage is transformed to acetaldehyde, the reaction order “a” can be considered to equal 0. This simplifies eq 5 to: r = k[O2]b or Ln(r ) = Ln k + b Ln[O2]

(6)

The rate is proportional to the peak area of acetaldehyde formation, and thus for the purpose of extracting the reaction order “b”, a plot of Ln[peak area] versus Ln[PO2] can be used. From Figure 9a, “b” was found to be equal to 0.86 (∼1.0). This may indicate that the reaction is pseudo-first-order and the formation of acetaldehyde is directly proportional to the O2 pressure within the investigated O2 pressure range. Figure 9b represents the rate of CO2 production during the same experiment as a function of background O2 pressure. It can be noticed that the rate of CO2 production increases systematically with the increase in O2 pressure, while in the case of TiO2 this trend has not been clearly observed. Using eq 6 for the CO2 production in Figure 9b, “b” was found equal to 0.89 (∼1.0). This may indicate that similar to acetaldehyde production the reaction is pseudo-first-order for CO2. Nadeem et al. previously reported that in the case of a similar reaction on TiO2(110) “b” was found equal to 0.5.27 This square-root dependence indicated that the formation of acetaldehyde was more sensitive to molecular oxygen at low pressures but saturated faster. However, the rate of acetaldehyde and carbon dioxide production from Ru/TiO2 surfaces increases with O2 pressures in a more linear trend. This indicates that more surface sites are available for the adsorption of O2, and these sites are most likely those at the interface of the Ru metal nanoparticles and TiO2 surfaces. By comparing the photo-oxidation of ethanol results in Table 1, it is evident

CH3CH 2OH + Ti s 4 + − Os 2 − → CH3CH 2O − Ti s 4 + + Os H(a) (ethoxide formation) TiO2 + UV → e− + h+

Table 1. Comparison of Acetaldehyde (m/z = 29) and Carbon Dioxide (m/z = 44) Amounts Produced during Photo-Oxidation of Ethanol on Predosed Rutile TiO2(110) and Ru/TiO2(110) at Saturation Coverage (2 L), 300 K, and in the Presence of UV Light with a Background O2 Pressure of 1 × 10−8 Torra

a

m/z

44

29

44/29

TiO2(110) Ru/TiO2(110)

4.88 × 10−4 5.47 × 10−4

0.34 × 10−4 2.09 × 10−4

14.35 2.61

(7a)

(electron excitation)

(7b)

CH3CH 2O − Ti s 4 + + h+ + Os → CH3C•HOTi s 3 + + Os H(a) (electron injection)

(7c)

CH3C•HOTi 3s + + O − Ru + e− → CH3CHO + Ti 3s + + O•−(a) + Ru

Amounts desorbed is in arbitrary units (uncalibrated Torr × time).

(acetalaldehyde formation)

that the Ru/TiO2 system is more active in photo-oxidation of ethanol to acetaldehyde than TiO2. The experimental results clearly indicate that although CO2 formation is similar in both systems the amount of acetaldehyde is far more in the case of the Ru/TiO2 system. This can be further understood by comparing the kinetics of photoreactions of ethanol to acetaldehyde4 and of acetates (from acetic acid38) to CO2. In separate experiments on TiO2(110) it was found that the cross section of ethanol4 reaction under UV excitation in the presence of oxygen is about one order of magnitude faster than that of acetates38 to CO2. This is in line with radical reactions in the gas phase (with OH radicals), where the second-order rate constant of ethanol39 reaction (3.7 × 10−12 cm3 molecule−1 s−1) is about six times faster than that of acetic acid (0.66 × 10−12 cm3 molecule−1 s−1).40 Thus, based on the observed amount of desorbed acetaldehyde in this work, the role of RuO2 deposited on TiO2(110) is to accelerate the first step (high k1). This is most likely due to the metallic character of RuO2, therefore

(7d)

2Os H(a) + Ti s 3 + + O•−(a) → Ti s 4 + + 2Os + H 2O (water formation) (7e)

where a means adsorbed; g means gas; and s means surface This can be explained as follows: ethanol adsorbs dissociatively forming ethoxides and surface hydroxyls (eq 7a). Upon UV exposure the electron and hole pairs are produced in TiO2 (eq 7b). Electrons are excited to the conduction band, leaving holes in the valence band. The electron in the conduction band may recombine with the holes or be trapped with Ru metal clusters present on the surface. The formation of a Schottky barrier at the metal(Ru)semiconductor(TiO2) junction helps to reduce the recombination probability and guides the transportation of holes due to an electrostatic potential at the interface.36 Ethoxide species may give an electron to the hole in the valence band and H



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convert to CH3C•HO(ads) (α-oxyethyl radical) (eq 7c). Oxygen is dissociatively adsorbed on Ru nanoparticles at room temperature.37 The chemisorbed oxygen can thus react with CH3C•HO(ads) present at the vicinity of the Ru metal to oxidize it into acetaldehyde (eq 7d). The O•− formed can react with two OH groups and one Ti3+ to produce water while converting each of the former O2−(surface) and Ti3+ into Ti4+ (eq 7e). Ethoxy radicals present away from the Ru metal react in the same way as on TiO2 alone to give acetaldehyde. However, in this case, O2 molecules are not dissociatively adsorbed but each O2 molecule reacts with one electron from the conduction band making O2−(radical). Because the formation of acetaldehyde requires two electron/hole transfers, there are two oxygen molecules consumed per one molecule of acetaldehyde. O2−(radical) may further react with the surface and with acetaldehyde to give CO2 as previously reported. Because the photocatalytic activity of chemisorbed oxygen is much higher than that for molecular oxygen, the overall reaction activity is mainly dictated by the former step.

DFT calculations were carried out using the computing facilities at the Center for Functional Nanomaterials at Brookhaven National Laboratory and National Energy Research Scientific Computing (NERSC) Center under contract no. DE-AC02-05CH11231.

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(1) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Current Status of Hydrogen Production Techniques by Steam Reforming of Ethanol: A Review. Energy Fuel 2005, 19, 2098−2106. (2) Idriss, H. Ethanol Reactions over the Surfaces of Noble Metal/ Cerium Oxide Catalysts. Platinum Met. Rev. 2004, 48, 105−115. (3) Murdoch, M.; Waterhouse, G. I. N.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; Llorca, J.; Idriss, H. The Effect of Gold Loading and Particle Size on Photocatalytic Hydrogen Production from Ethanol over Au/TiO2 Nanoparticles. Nat. Chem. 2011, 3, 489− 492. (4) Jayaweera, P. M.; Quah, E. L.; Idriss, H. Photoreaction of Ethanol on TiO2(110) Single-Crystal Surface. J. Phys. Chem. C 2007, 111, 1764−1769. (5) Sobczynski, A.; Jakubowska, T.; Zielinski, S. Hydrogen Photoevolution from Water-Methanol on Ru/TiO2. Monatsh. Chem. 1989, 120, 101−109. (6) Wetchakun, K.; Wetchakun, N.; Phanichphant, S. Enhancement of the Photocatalytic Performance of Ru-Doped TiO2 Nanoparticles. Adv. Mater. Res. 2008, 55, 853−856. (7) Wetchakun, K.; Phanichphant, S. Effect of Ru on Photocatalytic Activity of TiO2 Nanoparticles. J. Microsc. Soc. Thailand 2008, 22, 11− 14. (8) Sasirekha, N.; Basha, S. J. S.; Shanthi, K. Photocatalytic Performance of Ru Doped Anatase Mounted on Silica for Reduction of Carbon Dioxide. Appl. Catal., B 2006, 62, 169−180. (9) Yang, F.; Kundu, S.; Vidal, A. B.; Graciani, J.; Ramirez, P. J.; Senanayake, S. D.; Stacchiola, D.; Evans, J.; Liu, P.; Sanz, J. F.; Rodriguez, J. A. Determining the Behavior of RuOx Nanoparticles in Mixed-Metal Oxides: Structural and Catalytic Properties of RuO2/ TiO2(110) Surfaces. Angew. Chem., Int. Ed. 2011, 50, 10198−10202. (10) Kundu, S.; Vidal, A. B.; Yang, F.; Ramirez, P. J.; Senanayake, S. D.; Stacchiola, D.; Evans, J.; Liu, P.; Rodriguez, J. A. Special Chemical Properties of RuOx Nanowires in RuOx/TiO2(110): Dissociation of Water and Hydrogen Production. J. Phys. Chem. C 2012, 116, 4767− 4773. (11) Henderson, M. A. Photooxidation of Acetone on TiO2(110): Conversion to Acetate via Methyl Radical Ejection. J. Phys. Chem. B 2005, 109, 12062−12070. (12) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (13) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces - Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671− 6687. (14) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics for LiquidMetals. Phys. Rev. B 1993, 47, 558−561. (15) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (16) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (17) Graciani, J.; Nambu, A.; Evans, J.; Rodriguez, J. A.; Sanz, J. F. Au ↔ N synergy and N-doping of metal oxide-based photocatalysts. J. Am. Chem. Soc. 2008, 130, 12056−12063. (18) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W.; Smith, J. V. Structural Electronic Relationships in Inorganic Solids Powder Neutron-Diffraction Studies of the Rutile and Anatase

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CONCLUSIONS The presence of Ru metal and metal-oxides imposes changes in the electronic structure of TiO2; under UV light exposure, the mobility of electrons in the conduction band is significantly enhanced. Ethoxide is formed on both rutile TiO2(110) and RuOx-modified TiO2(110) surfaces when exposed to ethanol at room temperature. A fraction of the ethoxide species adsorbed on the TiO2 surface decomposes to acetaldehyde at high temperature. In contrast, in the case of RuO 2 /TiO 2 , acetaldehyde formation is less endothermic and can be obtained near 300 K. In this case, the formation of acetate species is highly favored for the produced acetaldehyde, where one of the bridging oxygen atoms from five-fold Ru sites is removed from the surface, and thus the RuO2 deposited on the TiO2 surface is partially reduced. Upon UV excitation and in the presence of O2, the main reaction product observed at 300 K is acetaldehyde on both bare TiO2(110) and on Ru metal/ metal-oxide modified TiO2 surfaces. This product, formed via a two-electron transfer process, requires the presence of molecular O2 to trap the excited electrons from the conduction band, and thus decreases the rate of electron−hole recombination. It is found that the RuOx/TiO2 system is more active for the photo-oxidation of ethanol to acetaldehyde than TiO2. A linear trend of the rate of acetaldehyde and carbon dioxide production from RuOx/TiO2 with O2 pressures indicates that more surface sites are available for the adsorption of O2 than on bare TiO2 surfaces, possibly at the interface of the Ru nanoparticles and TiO2 surfaces, which facilitates the photooxidation.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 631-344-4378. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research carried out at Brookhaven National Laboratory was done under contract no. DE-AC02-98CH10886 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences, and Biosciences within the Office of Basic Energy Sciences. The I



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Polymorphs of Titanium-Dioxide at 15 and 295-K. J. Am. Chem. Soc. 1987, 109, 3639−3646. (19) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (20) Kim, Y. K.; Hwang, C. C. Photoemission Study on the Adsorption of Ethanol on Clean and Oxidized Rutile TiO2 (110)-1 × 1 Surfaces. Surf. Sci. 2011, 605, 2082−2086. (21) Kim, Y. K.; Kay, B. D.; White, J. M.; Dohnalek, Z. Alcohol Chemistry on Rutile TiO2(110): The Influence of Alkyl Substituents on Reactivity and Selectivity. J. Phys. Chem. C 2007, 111, 18236− 18242. (22) Chen, J. G. NEXAFS Investigations of Transition Metal Oxides, Nitrides, Carbides, Sulfides and Other Interstitial Compounds. Surf. Sci. Rep. 1997, 30, 1−152. (23) Resta, A.; Blomquist, J.; Gustafson, J.; Karhu, H.; Mikkelsen, A.; Lundgren, E.; Uvdal, P.; Andersen, J. N. Acetate Formation during the Ethanol Oxidation on Rh(111). Surf. Sci. 2006, 600, 5136−5141. (24) Gamble, L.; Jung, L. S.; Campbell, C. T. Decomposition and Protonation of Surface Ethoxys on TiO2(110). Surf. Sci. 1996, 348, 1− 16. (25) Hansen, J. O.; Huo, P.; Martinez, U.; Lira, E.; Wei, Y. Y.; Streber, R.; Laegsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Direct Evidence for Ethanol Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 2011, 107, 136102. (26) Farfan-Arribas, E.; Madix, R. J. Role of Defects in the Adsorption of Aliphatic Alcohols on the TiO2(110) surface. J. Phys. Chem. B 2002, 106, 10680−10692. (27) Nadeem, A. M.; Muir, J. M. R.; Connelly, K. A.; Adamson, B. T.; Metson, B. J.; Idriss, H. Ethanol Photo-Oxidation on a Rutile TiO2(110) Single Crystal Surface. Phys. Chem. Chem. Phys. 2011, 13, 7637−7643. (28) Thomas, A. G.; Syres, K. L. Adsorption of Organic Molecules on Rutile TiO2 and Anatase TiO2 Single Crystal Surfaces. Chem. Soc. Rev. 2012, 41, 4207−4217. (29) Li, Z. J.; Smith, R. S.; Kay, B. D.; Dohnalek, Z. Determination of Absolute Coverages for Small Aliphatic Alcohols on TiO2(110). J. Phys. Chem. C 2011, 115, 22534−22539. (30) Huo, P.; Hansen, J. O.; Martinez, U.; Lira, E.; Streber, R.; Wei, Y. Y.; Laegsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Ethanol Diffusion on Rutile TiO2(110) Mediated by H Adatoms. J. Phys. Chem. Lett. 2012, 3, 283−288. (31) Wilson, J. N.; Idriss, H. Effect of Surface Reconstruction of TiO2(001) Single Crystal on the Photoreaction of Acetic Acid. J. Catal. 2003, 214, 46−52. (32) Muir, J. N.; Choi, Y.; Idriss, H. Computational Study of Ethanol Adsorption and Reaction over Rutile TiO2 (110) Surfaces. Phys. Chem. Chem. Phys. 2012, 14, 11910−11919. (33) Plata, J. J.; Collico, V.; Marquez, A. M.; Sanz, J. F. Understanding Acetaldehyde Thermal Chemistry on the TiO2 (110) Rutile Surface: From Adsorption to Reactivity. J. Phys. Chem. C 2011, 115, 2819−2825. (34) Zehr, R. T.; Henderson, M. A. Acetaldehyde Photochemistry on TiO2(110). Surf. Sci. 2008, 602, 2238−2249. (35) Feng, W.; Wu, G. J.; Li, L. D.; Guan, N. J. Solvent-Free Selective Photocatalytic Oxidation of Benzyl Alcohol over Modified TiO2. Green Chem 2011, 13, 3265−3272. (36) Uetsuka, H.; Pang, C.; Sasahara, A.; Onishi, H. Photochemical Reaction of Trimethyl Acetate on Pt/TiO2(110). Langmuir 2005, 21, 11802−11805. (37) Parrott, S. L.; Praline, G.; Koel, B. E.; White, J. M.; Taylor, T. N. Oxygen-Chemisorption on a Stepped Ru (Approximately 001) Crystal. J. Chem. Phys. 1979, 71, 3352−3354. (38) Quah, E. L.; Wilson, J. N.; Idriss, H. Photoreaction of the Rutile TiO2(011) Single-Crystal Surface: Reaction with Acetic Acid. Langmuir 2010, 26, 6411−6417. (39) Butkovskaya, N. I.; Kukui, A.; Pouvesle, N.; Le Bras, G. Rate Constant and Mechanism of the Reaction of OH Radicals with Acetic

Acid in the Temperature Range of 229−300 K. J Phys Chem A 2004, 108, 7021−7026. (40) Alam, M. S.; Rao, B. S. M.; Janata, E. (OH)-O-Center Dot Reactions with Aliphatic Alcohols: Evaluation of Kinetics by Direct Optical Absorption Measurement. A Pulse Radiolysis Study. Radiat. Phys. Chem. 2003, 67, 723−728. (41) Reztsova, T.; Chang, C. H.; Koresh, J.; Idriss, H. Dark- and Photoreactions of Ethanol and Acetaldehyde over TiO2/Carbon Molecular Sieve Fibers. J. Catal. 1999, 185, 223−235.

J


Ethanol Photoreaction on RuO - American Chemical Society

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